1. Field of the Invention
The present invention relates to a multi-band plane inverted-F antenna (PIFA), especially to a multi-band PIFA with an inverted-L short-line to induce a loop surface current, thus enhancing the bandwidth of the multi-band PIFA at multiple frequencies of interest.
2. Description of Prior Art
Modern electronic products for consumer have the trends of compact size and light weight, as manufacture technology and design skill progress. It is also desirable to integrate more versatile functions to the electronic products. Taking wireless communication devices as example, the networking accessing function through wireless application protocol (WAP) and e-mail function are augmented to the original voice function.
Moreover, smart phones with data processing ability and wireless LAN function are also attractive to user these years. The smart phones can provide advanced functions such as mobile TV and business transaction as the wireless bandwidth increases and the processing ability of mobile phone is enhanced. To this end, the antenna for mobile phone is also demanded to scale down for compact requirement.
The physical size of a microwave antenna, such as dipole antenna and microstrip patch, is around the half wavelength of the resonant modes of the antenna size. To further minimize the physical size of a microwave antenna, planar inverted-F antenna (PIFA) is developed to provide signal transmission/reception at quarter wavelength. Planar inverted-F antenna can also be realized as a hidden antenna for mobile phone due to the low profile property.
FIG. 1 shows the schematic diagram of a prior art planar inverted-F antenna. The planar inverted-F antenna 10a mainly comprises a radiating unit 12a, a ground plane 20a, a dielectric material (not shown), a shorting element 16a and a feeding element 14a. The dielectric material is sandwiched between the radiating unit 12a and the ground plane 20a to provide isolation therebetween. The radiating unit 12a is coupled to the ground plane 20a through the shorting element 16a. The feeding element 14a is arranged on the ground plane 20a and is coupled to the radiating unit 12a for signal transmission. The radiating unit 12a and the ground plane 20a can be implemented with metallic material. The radiating unit 12a is designed with specific pattern for achieving desired operating wavelength and radiation performance. The most attractive feature of planar inverted-F antenna is the ability to work at quarter wavelength for advantageously reducing the size of antenna.
However, the prior art planar inverted-F antenna has the drawbacks of insufficient bandwidth and inability to work at multiple frequencies (more than dual-band frequencies). The smart phone is expected to work at tri-bands or even qua-band mobile communication frequencies, and have accessing ability to WLAN. Therefore, it is important issue to provide a multi-band planar inverted-F antenna for mobile phones such as smart phones.